Inks and coatings are used for printing text and images on a variety of different surfaces (or substrates), ranging from cellulosic articles such as paper and corrugated cardboard, plastic articles and films, metals, to items of clothing. Ink is generally applied to a portion of the substrate in order to produce a detailed image such as letters or a picture. A coating is generally applied uniformly over an entire substrate as a continuous film. Inks and coatings that are subject to rubbing and scuffing, either in transit, storage or use, can become streaked with lines and, in the case of text, become difficult to read, or lose the image quality expected by end users.
The ultimate use of the ink has a role in determining its formulation, because certain properties are more desirable in one type of an ink than another. Various types of ink are used in processes such as high speed newspaper printing, gravure inks, flexography, thermal offset, heatset, letter press, screen printing, spraying, brushing, or the like.
For the purpose of the present invention the term flexographic ink shall refer to the ink used in flexography, flexography being defined as “. . . us[ing] raised rubber or photopolymer plates (for printing), and requires far shorter make-ready than letterpress or offset printing . . . Flexography is used for printing milk cartons, narrow-web labels, flexible packaging, corrugated board and paperback books”. (A. Glassman, 1985, Printing Fundamentals, TAPPI Press, p. 322). In addition, flexography can be used for printing on other substrates, such as plastic films, foils, coated and uncoated papers, and paperboard.
Additives are often incorporated into inks and coatings by being mixed or ground into the ink or coating formulation with pigments; added as a part of the final ink blend; or introduced at other times. Printing inks in particular utilize such additives, so that the ink will not rub off when the substrate surface is subjected to the normal abrasive forces encountered in use and handling, such as during handling of paper products, or during shipment of corrugated cartons under the moist conditions that accompany the handling of refrigerated or frozen goods, or goods shipped packed in ice. Addition of selected additives often also yields improved slip properties. Slip properties permit other printed pages to contact and rub over the ink or coating without causing the ink to smudge. Controlled slip is often desirable for such articles as magazines that are to be stacked; it would be undesirable to have the stack slide apart. One way of controlling the slip property is utilization of micronized waxes incorporated into the ink or coating. It is theorized that the underlying mechanism involves some interlocking of wax particles between the two coated surfaces. The wax is micronized, and dispersed in the formulation to such a degree that it cannot be detected by the human eye. Dispersion of micronized waxes into the formulations also has a lesser effect on gloss reduction than if the wax were melted into the ink or coating formulation. It is known in the art that micronized waxes can render inks and coatings less susceptible to abrasion and also enable control of slip.
Some waxes used in ink and coating formulations to modify mar abrasion and slip modification are often supplied in powder form. Hard waxes are generally jet milled to a particle size ranging between approximately 5 and approximately 15 microns, with the resultant products often being referred to as ‘micronized waxes’. These powdered waxes are often difficult to disperse in ink or coating formulation because of high surface tension and the need to break apart agglomerates. To facilitate their dispersion in ink or coating formulations, suppliers of micronized waxes often surface treat the powder with surfactants or other dispersing aids, or mill the powder into a paste that is more readily dispersed into the final ink or coating formulation.
The jet milling of wax into micronized particle sizes is an energy intensive process which also generates heat. This process may often cost more than the value of the wax itself. Harder waxes, defined as those having a hardness of less than 6 dmm (as measured by the needle penetration test for wax hardness) jet mill easier than softer waxes, and therefore cost less to micronize. The melting point of the wax also affects the ability of a wax to be micronized; if the melting point is too low, the heat generated in the jet milling may cause the wax to melt, rendering the process incapable of micronizing the wax. Addition of mechanical chillers to jet mills has been utilized, but because it increases milling costs still further, it is generally considered not to be cost effective.
Many commercially available anti-abrasion compounds contain polytetrafluoro-ethylene (“PTFE”) in the form of a micronized powder. PTFE, commercially available from E.I. DuPont, Wilmington, Del. and sold under the trademark TEFLON®, has a low surface energy making it difficult to disperse and requiring long mixing times. One such PTFE-containing compound is Protech 120, sold by Carroll Scientific, Inc. (a division of Lubrizol Corp, Wickliffe, Ohio) which is described as a high solids (approximately 83%) virgin PTFE wax compound, and is used in ink formulations.
Polyethylene waxes have also been used as anti-abrasion additives in the ink industry. The ink manufacturer normally incorporates these waxes as dispersions of the wax in resins, generally of the same type as the ink formulations into which they are to be incorporated.
Murayama, in U.S. Pat. No. 3,843,570 describes a porous material comprising PTFE and having micropores, which material is obtained by polymerizing a monomer capable of forming a thermoplastic resin. The resultant material is suitable with inks and is also printable.
U.S. Pat. No. 5,158,606 (Carlick et al.) describes a printing ink composition with a high degree of rub-off resistance comprising a dispersion of a pigment in a vehicle containing a C7-C40 oil and a polymer latex emulsified in the dispersion. The inventors state that synthetic waxes, such as the polyethylene or PTFE waxes, are the most popular ones used in the ink industry. They also indicate that the relative cost of PTFE waxes is prohibitively high for applications such as news inks, but where cost is not of paramount concern, a PTFE wax with petrolatum can be added to the oil/polymer latex ink composition.
Mueller et al. (U.S. Pat. No. 5,643,984) disclose a wax composition for the printing ink industry which utilizes polyethylene waxes and oxidized polyethylene waxes; Fischer-Tropsch waxes; microcrystalline or carnauba waxes in an ink formulation based on either an aromatic or aliphatic solvent.
The prior art illustrates the use of petroleum-derived waxes and synthetic waxes for formulating ink and coating compounds. There are no mentions of vegetable derived waxes for use in ink and or coating formulations, yet there is a recognized and long-felt need to find alternatives to products such as PTFE, expensive synthetic waxes such as microcrystalline waxes, and other petroleum waxes that are derived from increasingly scarce and limited natural resources. There is also a recognized and long-felt need to use materials in inks and coatings that are considered safe to humans because of the ink or coating's use in the manufacture of paper and plastic packaging used to transport and store foodstuffs. There is also a recognized and long-felt need to use materials in ink and/or coatings that are naturally derived and can be easily recycled back into the environment without long-term adverse effects; consumer packaged goods, for example, are known to be difficult to recycle. Therefore, there is a need for employing a wax, which has similar properties of commercially available PTFE, and petroleum derived or synthetic waxes used in ink and coating formulations. Due the large volume of waxes consumed in these applications it is also preferred that the compositions be readily available. From both a supply and a natural resource viewpoint, it is preferred that the compositions be obtained from a source that preferably is renewable, such as from plant extracts.
There is a need for a wax that is hard and high melting and can be micronized into powder cost effectively. Given that the world's petroleum supply is finite, and dwindling, it is also desirable to have a wax that can be obtained from a renewable source, such as plants, rather than being petroleum based.
The anti-abrasion properties of the waxes of the present invention are most useful for water based inks and coatings and are particularly suited for use in flexographic inks. These waxes are particularly well suited for rendering inks and coatings less susceptible to mar and abrasion because they are very hard relative to most conventional petroleum based waxes and they are derived from renewable natural resources.